Method for determining concentration profiles of deep levels on both sides of a p-n junction

ABSTRACT

The deep level (&#39;&#39;&#39;&#39;trap&#39;&#39;&#39;&#39;) impurity concentration, as a function of distance from a P-N junction in a semiconductor, is determined by a modified photocapacitance technique. This technique utilizes a sequence of operations involving a source of incident monochromatic optical radiation of different wavelengths (for emptying or filling a predetermined proportion of the deep levels of their captured charge carriers) and involving a cycling of a reverse voltage bias applied to the junction. The difference in electrical capacitance of the junction as measured before and after the optical irradiation of the junction, as a function of reverse voltage bias, is a measure of the concentration of deep levels as a function of distance from the P-N junction. The deep level concentration on both the P and the N side of the junction can thereby be determined.

United States Patent [191 Henry et al.

[451 July 24, 1973 METHOD FOR DETERMINING CONCENTRATION PROFILES OF DEEPLEVELS ON BOTH SIDES OF A P-N JUNCTION [75] Inventors: Charles Hoyy ardHenry, New

Providence; Hiroshi Kukimoto, North Plainfield, both of NJ.

[73] Assignee: Bell Telephone Laboratories,

Incorporated, Murray Hill, NJ.

221 Filed: Nov. 12,1971

21 Appl. No.: 198,321

[56] References Cited OTHER PUBLICATIONS Spiwak; R. R., Design and IEEETrans. On Inst. and Meas., Vol. IM-l8, No. 3, September 1969, pg.197-202.

Williams, R., Determination of Journal of App. Physics, Vol. 37, No. 9,August 1966, pg. 3411-3416.

Copeland, J. A., A Technique IEEE Trans. on

Elect. Devices, Vol. ED-16, No. 5, May 1969, pp. 445-449.

Sah et al., Thermal and Solid State Electronics, Vol. 13, June 1970, pp.759-785.

Primary Examiner-Rudolph v. Rolinec Assistant Examiner- Emest F. KarlsenAttorneyW. L. Keefauver [57] ABSTRACT The deep level (trap) impurityconcentration, as a function of distance from a P-N junction in asemiconductor, is determined by a modified photocapacitance technique.This technique utilizes a sequence of operations involving a source ofincident monochromatic optical radiation of different wavelengths (foremptying or filling a predetermined proportion of the deep levels oftheir captured charge carriers) and involving a cycling of a reversevoltage bias applied to the junction. The difference in electricalcapacitance of the junction as measured before and after the opticalirradiation of the junction, as a function of reverse voltage bias, is ameasure of the concentration of deep levels as a function of distancefrom the P-N junction. The deep level concentration on both the P andthe N side of the junction can thereby be determined.

10 Claims, 2 Drawing Figures CAPACITANCE METER METHOD FOR DETERMININGCONCENTRATION PROFILES OF DEEP LEVELS ON BOTH SIDES OF A PN JUNCTIONFIELD OF THE INVENTION This invention relates to methods for testingsemiconductors, in particular, for determining the concentration profileof deep trap levels lying within the energy gap of a semiconductor.

BACKGROUND OF THE INVENTION In luminescent semiconductor devices, withinthe forbidden energy band gap of the semiconductor, ordinarily there arepresent certain levels (traps) lying deep in this gap. By deep is meantsignificantly greater in energy difference from the valence orconduction energy band edge than the shallow" levels (associated-withacceptors or donors). In P-N electroluminescent devices'(light emittingdiodes, for example), these deep lying levels can function asnonradiative recombination centers for the injected minority carriers(holes in N-type, electrons in P-type semiconductor). Thereby, suchtraps in the neighborhood of a PN junction of a light emitting diodetend to reduce the luminescent efficiency of the diode.

More particularly, in one type of red light emitting PN junctionsemiconductive gallium phosphide diode, the emission of red light occursfrom zinc-oxygen molecular type pairs in the neighborhood of the PNjunction. Typically, in such a diode there are unavoidably presentunpaired oxygen atoms in this neighborhood of the PN junction, theseatoms functioning as traps in the P zone which enable injected electronsto recombine with holes nonradiatively (or else radiatively at anunwanted wavelength). Thereby, these oxygen traps compete with thezinc-oxygen pairs for these injected electrons. Thus, these oxygen trapsreduce the number of injected electrons available for the desiredcapture and radiation by the zinc-oxygen pairs; therefore, these trapsreduce the quantity of desired emitted light.

In the fabrication of electroluminescent semiconductive diodes, as amatter of quality control, it is therefore important to know theconcentration of traps and where they are located (i.e., concentrationprofile) particularly with reference to the PN junction. In the US. Pat.application Ser. No. 3,771 by J. A. Copeland, filed in the US. on Jan.19, 1970, apparatus and methods are described for determining the deeplevel trap profile in a semiconductor in the neighborhood of a Schottkybarrier (i.e., junction of a semiconductor with a method). However, sucha technique is not readily adaptable for determining the trap levelprofile in the neighborhood of a PN junction on both the I and the Nside thereof.

Accordingly, it would be desirable to have a method for determining thetrap level profile on either side of a P-Njunction.

SUMMARY OF THE INVENTION This invention is based on our discovery thatthe incremental contribution to the electrical capacitance (AC) of a P-Njunction in a semiconductor body due to trap levels is proportional tothe concentration of those (and only those) trap levels which are filledwith charge carriers in the neighborhood of the PN junction. Moreparticularly, we have devised a method for filling only a knownproportion of the total trap levels occupied with charge carriers; andwe have also therebydevised a method for determining the trap levclconcentration at the known depth in this neighborhood of the PNjunction. More particularly, in accordance with the invention, the knowntrap levels are selectively filled, to a given proportion in givenneighborhoods of the PN junction, by means of a combination of stepsinvolving suitable optical irradiation and manipulation of reverse biasvoltage. In manipulating the reverse bias voltage, in accordance with afeature of the invention, the magnitude of the reverse bias is reducedfrom a value V to a value V and then cycled back to the value V Thereby,all trap levels are filled on the N side of the junction in the regionbetween the edges of the depletion regions on the N side correspondingto V and V; whereas all trap levels are emptied on the P side of thejunction in the region between the corresponding edges of the depletionregion on the P side of the junction; the trap levels elsewhere beingunaffected by this cycling of reverse bias.

In a specific embodiment of this invention, for measuring the trap levelconcentration on the N side of the PN junction, the junction issubjected to a reverse bias voltage (V,,) sufficient to produce adepletion region (shallow levels ionized) which includes the location(s)at which the trap level concentration is desired to be determined. Whilemaintaining this bias voltage, the junction is irradiated with opticalradiation having a wavelength suitable (sufficiently low photon energy)for emptying all trap levels within the depletion region (except forapproximately five Debye lengths from the two opposite edges of thedepletion region, where the trap level occupations are unaffected by anyirradiation). Then, in the absence of the optical radiation, the reversebias voltage is decreased to a value V such that the edge position (x)of the depletion region moves to a preselected location (on the N sideof the junction) where it is desired to know the trap levelconcentration. Then, immediately thereafter the bias voltage is returnedto the original reverse bias voltage, V Thereby, all the trap levels onthe N side of the junction are filled from the edge of the depletionregion to the preselected location, but the trap levels on the P sideremain empty as before. The electrical capacitance (C,) of the body (dueto the depletion region of the junction) is then measured, andthereafter again the junction is irradiated with optical radiation .toempty all the trap levels (with the voltage bias at V Finally, thecapacitance (C,) of the body (junction) is again measured (with thevoltage bias at V The difference in capacitance (C -C the incrementalcapacitance (AC), is then determined. It should be understood thatcommercial capacitance measuring devices (such as Boonton Model 71Capacitance Meter) are available to monitor directly this difference AC.The whole sequence of steps is repeated for a slightly different valueof V, and hence for a slightly different value (x) of the edge positionof the depletion region, together with a slightly different resultingvalue of AC. The spatial rate of change of this incremental capacitance,d(AC)/dx, can then be determined and is proportional to the total traplevel concentration at this position x on the N side of the junction.

In carrying out any sequence of steps for measuring the incrementalcapacitance (C -C according to this invention, it is important that thetemperature of the semiconductor body be maintained at a sufficientlyconstant value so that any variations in electrical capacitance, due tovariations in thermal ionization of shallow donor or acceptor levels, bekept negligible as compared with variations in capacitance due to thefilling and emptying of trap levels.

In another specific embodiment of the invention, adapted for measuringthe trap level concentration on the P side of a P-N junction, the traplevel concentration on the N side of the junction is first measured asdescribed above. To every position on the N side, there corresponds aposition on the P side, to wit, the edge of the depletion region on theP side when the junction is subjected to the reverse bias voltage V. Inorder to measure the trap level concentration on the P side at thiscorresponding position, the spatial rate of change of incrementalcapacitance (dAC/dx) is again measured similarly as described before forthe N side, but now using an irradiation having a different opticalwavelength, so that a known fraction (nonzero) of the trap levels areemptied on both the N and P sides of the junction. The cycling of thereverse voltage bias (V,, to V to V,,) now empties all trap levels in aregion on the P side of the junction, in addition to filling all traplevels in a region on the N side of the junction. Thus, a weightedproportion of the spatial rates of changes of incremental capacitanceusing the different optical wavelengths yields the trap levelconcentration on the P side of the junction.

BRIEF DESCRIPTION OF THE DRAWING This invention together with itsfeatures, advantages, and objects may be better understood from thefollowing detailed description when read in conjunction with the drawingin which:

FIG. I is a schematic diagram of a circuit useful for carrying out theinvention; and

FIG. 2 is a schematic representation of a P-N junction useful fordescribing the theory of operation of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS FIG. I shows a diagram ofapparatus suitable for measuring the deep trap level concentrationprofile in the neighborhood of a P-N junction 10 in a semiconductor body111; for example, oxygen traps in a semiconductive gallium phosphide.The body It is contacted by a pair of electrodes, 10.11 and 10.2, whichare electrically connected to a battery 12 and a capacitance measuringdevice 113, typically a Boonton capacitance meter, which can readdirectly any changes in capacitance as well as absolute values ofcapacitance. The polarity of the battery I2 is arranged to provide areverse bias voltage across the P-N junction H0. An optical source 21furnishes optical radiation incident on the body I0, of wavelengths A,and A Preferably, the optical radiation of wavelength A, (or a narrowspectral range containing I is selected such that the probability forhole emission (by the deep level traps to be measured) is equal to zero(e but e is not zero; whereas A, (or a narrow spectral range) isselected such that e is not equal to zero. For measuring deep leveloxygen traps in gallium phosphide, A, is selected in a range betweenabout 9,500 angstroms and 13,500 angstroms; whereas A 2 is selected in arange between about 7,300 angstroms and 8,300 angstroms, typically about8,000 angstroms.

The preferred sequence of steps for measuring the incrementalcapacitance (and hence the concentration of deep levels therefrom) areas follows: 1. Set reverse bias voltage 12 at V,,. 2. Irradiate body 11with k, (or M) from optical source 21.

3. Block off irradiation- 4. Cycle reverse bias voltage 12 from V to Vand back to V 5. Measure capacitance C, of junction 10 with capacitancemeter 13..

6. Irradiate body 11 with optical source 21.

7. Measure capacitance C ofjunction 10 with capacitance meter 13.

Thereby, the incremental capacitance, i.e., the difference (C,C AC, canbe determined. With available capacitance meters, this difference AC canbe measureddirectly without separately measuring C, and C Thereafter,steps I through 7 are repeated using a slightly different value for thereverse bias voltage V; thence d(AC)/dx is determined; and finally theconcentration of deep levels are determined as explained below,according toequation (5) or (9). By slightly different is meant anincrement sufficiently small that the resulting linear approximation ofdifferent quotients approximates the derivative d(AC)/d.x as closely asdesired.

In carrying out any sequence of steps 1 through 7 above, it is importantto maintain the temperature of the body at a relatively constanttemperature, particularly during the time between the measurements of C,and C so that variations in the incremental capacitance (AC C,C due tovariations in the thermal ioniza tion of shallow (donor or acceptor)levels are negligible. Advantageously, for gallium phosphide containingdeep oxygen trap levels, this temperature is kept constant to within i01 C. Likewise, the temperature should be kept low enough, and themeasurements of C, and C performed quickly enough, to preventsignificant changes in the charge carrier occupations of the deep levelsdue to thermal excitation during and between each pair of measurementsof C, and C Again, for gallium phosphide containing the oxygen levels, atemperature of about K is useful, although room temperature can also beused.

In order to measure the trap level concentration on the N side of theP-N junction 10, the following details are advantageously observed incarrying out steps I through 7 above. The reverse bias voltage suppliedby the battery 12 is first set to a value V (step I), typically 10volts, sufficient to create a depletion region in the neighborhood ofthe P-N junction 10 which is at least as wide to include all points (x)at which the concentration of deep levels is to be measured. In terms ofthe drawing (FIG. 2), this reverse bias voltage V,, produces a depletionregion in the neighborhood of the junction 1-0, running from x 0 to x 1.While maintaining this bias voltage V,,, the body 10 is irradiated (step2) with optical radiation A, from the source 21, sufficient to empty thecharge carriers from all trap levels between x 0 and x 1 (except forapproximately five Debye lengths, x,,, immediately adjacent to x 0 and x1). Within the regions located from x 0 to x x,,, and from x lx,,) to x1, there are sufficient concentrations of free charge carriers(electrons and holes) that is not possible to change the occupationnumbers of the deep trap levels merely by means of optical radiation.Typically, an irradiation of about 2 X 10 watts/cm of wavelength betweenabout 9,500 angstroms and 13,500 angstroms for 100 seconds or more issuitable. Then the optical radiation is blocked (step 3) from the body10, and the bias voltage 12 is quickly reduced in magnitude to a value Vwhich is less than V and immediately returned back to its original valueV (step 4).

As a consequence of the reduction of the bias voltage to the value V,the depletion region is reduced in size by an amount x (FIG. 2) on the Nside of the P-N junction 10 and by an amount of distance x on the P sidethereof; and then with V again, the depletion region returns to itsoriginal width.(see curved arrows in FIG. 2). The temporary reduction ofbias voltage to V brings the edge of the depletion region on the N sideto x x and to x (l-x,,) on the P side. The distances 2: and x, can bedetermined by known methods, such as described below. In any event, thisconsequent reduction of the width of the depletion region advantageouslyresults in a value of x corresponding to a position at which it isdesired to measure the trap level concentration; and this movement ofthe edge of the depletion region fills the trap levels situated only inthe region from x 1: to x (x +x Then, after the bias voltage supplied bythe battery 11 is returned to the earlier value V the electricalcapacitance C of the P-N junction 10 is measured (step 5) by thecapacitance meter 13. Thereafter (with V still applied), the body 11 isagain irradiated (step 6) by the optical source 21, which empties thetrap levels from x 1: to x (x-+x,,) (except for about the aforementionedfive Debye lengths adjacent at 0). Again, the capacitance C, of thejunction is measured (step 7), and the incremental capacitance AC C -Cis thus determined. (With commercially available apparatus, thisincremental capacitance can be measured directly by monitoring thecapacitance as the junction capacitance goes from C to C Theabove-described steps 1 through 7 of capacitance measurements are thenagain performed, but with a slightly different voltage V, thus yieldinga slightly different capacitance incremental capacitance AC The spatialrate of change, d(AC-)/dxis proportional to the deep trap levelconcentration at x (x +x,,); therefore, it only remains to determine xas a function of V.

In order to calculate the'value of x corresponding to the reverse biasvoltage V, the following steps can be carried out. The (total)capacitance C( V of the junction 10 is measured under the reverse biasvoltage V,,; and the (total) capacitance C( V) of this junction is alsomeasured under the lower reverse bias voltage V. The contribution tocapacitance due to deep trap levels is ordinarily negligible as comparedwith the contribution to the capacitance due to the net significantshallow donor level concentration N on the N side of the P-N junction 10and the net significant shallow acceptor level concentration N on the Pside of this junction. Therefore, it follows from conventional theorythat:

Knowing the concentration of net significant donors N,,(x-) on the Nside of the junction, and net significant acceptors N 0,.) on the Pside, it likewise follows from theory that It should be understood thatboth N and N can be functions of x, although it is often a goodapproximation that their ratio N /N is constant; and in any event, N andN can be determined by known techniques, such as set forth in Appendix Abelow for example.

Although equations (1) and (2), in general, would require numericalintegration, these equations can be simply solved for the case ofconstant (N /N ratio, to wit,

N( D/ A) 1 VR)/C( and P( A/ D) 1 VR)/C( From equations (3.1) and (3.2),x and qr can be determined as a function of reverse bias voltage V, asdesired for correlating the measurements of AC as a function of x.

Having thus correlated the measurements of AC as a function of x thevarious values of AC serve as a basis for determining the concentrationN,(x) of deep trap levels as a function of x. In order to compute thisN,(x), it is first to be noted that for a concentration n,(x), of filleddeep trap levels, their contribution to the incremental capaictance is:

(The empty deep trap levels do not contribute to the capacitance.)Therefore, in the above-described sequence of steps for determining AC(remembering that the optical radiation affected only those deep levelssituated a distance of 1: or more from the P-N junction), it followsthat N,(x) is effective in the integrandof equation (4), as a filledlevel n,(x), only between x x and x x x hence, difi'erentiating equation(4) with respect to x (in the upper limit of the integral) yields:

Thus, from equation 5 the deep level trap concentrationN, at (x +x-) canbe calculated from the knowledge of the other parameters. However, asproceeded thus far, only N,(x) on the N side of the junction 10 can befound.

In order to determine the trap concentration N, on the P side of thejunction 10, it is necessary to proceed further with steps 1 through 7above, but with optical radiation A, Again, the bias voltage 11 is setat V and the body 11 is irradiated with optical radiation A, from thesource 21. In this case, A, is selected such that a known nonzero ratioof deep trap levels are filled all the way from x 1: to x lx Forexample, for oxygen levels in gallium phosphide, A, is advantageously inthe range between about 7,300 Angstroms and 8,300 Angstroms. Thus, thefilled trap density will be given by:

where e and e are the known hole and electron emission rates for deeptrap levels irradiated with the optical radiation of wavelength M. (Fora method to determine the ratio, r e /(e,,,+e see Appendix B below.)Then, with the traps filled to the extent n, given by equation (6), allirradiation is removed from the body 11, and the bias voltage 12 iscycled from V to V and back to V As a consequence, this cycling ofvoltage now has rendered empty all deep trap levels in the region (1x to(l-x -x but renders completely filled all those deep trap levels lyingin the region extending from x,, to (x +x-), and leave only partiallyfilled in the proportion e /(e-+e all those deep .trap levels from (x+xto (lx x In this condition of trap level ionization profile, thecapacitance C, of the body is measured by the capacitance meter 13.Again, the body 10 is irradiated with A, from the optical source 21,thereby partially filling to the extent (e /e,,+e the deep level trapssituated from (lx,,x to (lx and likewise rendering only partially fullto the extent e l- (e-+e,) those deep trap levels located from x,, to (x+x-). Once again, in this last condition of partial trap ionization, thecapacitance C of the body 10 is measured by the capacitance meter 13.With available apparatus, the change in capacitance (Cy-C can bemonitored directly. This increment A'C C C, has two contributions; onefrom the reduction in filled trap levels on the N side of the junction,the other contribution from the increase in filled trap levels on the Pside of the junction (both due to the preceding step of opticalirradiation). Thus, A'C A'C AC as follows:

It therefore follows from equation (4), and the discussion following it,that:

A'C lr)AC where A refers to the measurements with optical wavelength A,,and A refers to the earlier described measurements with M. Taking thespatial derivative of A'C with respect to x,:

l NtU ll' v)l( -':vo----w..)Nxli'i w..)/N|,l

Knowing the ratio r at wavelength A and knowing AC as a function of xfrom the earlier measurements (with M), and A'C as a function of x,(with fixed A, and various values for V, and hence as a function of x itis now straightforward to calculate N, at x (lx x from equation (9).That is, the concentration of deep trap levels N, at locations on the Pside of the junction 10 in the body 1 1 can now be calculated fromequation (9), as desired in this invention.

It should be remembered that it is important that the temperature of thebody 11 should be kept constant during all steps of this invention,advantageously to within 1- 0.1 l(, in order that variation in thermalionization of shallow levels be kept negligible as compared with theknown emission rates, e and 8p, of the deep levels induced by the.optical radiation from the source 21. For deep oxygen trap levels ingallium phosphide, this temperature is advantageously about K, but canbe room temperature.

As an alternative to the above recited order of sequence of steps, it ispossible to carry them out, at some sacrifice of accuracy, in thefollowing order of sequence:

A. Set reverse bias voltage 11 at V B. lrradiate body 10 with A, (or M)from optical source 21. C. Measure capacitance (C,) of junction 10 withcapacitance meter 13. D. Block off irradiation. E. Cycle reverse biasvoltage 11 from V to V and back to V F. Measure capacitance (C,) ofjunction 10 with capacitance meter 13. Then the capacitance difference(Cy-C AC is determined, or else AC determined directly by monitoring thecapacitance change with available apparatus; and the steps A through Fare repeated using a slightly different value for the reverse biasvoltage V, in order'to find d(AC)/dx, just as discussed above inconnection with steps 1 through 7. This sequence, A through F, however,suffers from some sacrifice of accuracy as compared with the preferredsequence 1 through 7 described earlier, due to errors arising in ACattributable to the difficulty in setting the maximum reverse biasvoltage back at precisely the same value V after cycling back from thelower value V; whereas in the preferred sequence of steps 1 through 7,any such errors arising in AC are minimized.

It should be understood that the sequences of steps for determining theconcentration profile of deep level traps from the capacitances C and C,can be automated. For example, the outputs of the capacitance meter 13can be in the form of electrical signals which are directly proportionalto the incremental capacitances (AC=C,C corresponding to the voltagebiases V, and hence to distances 1,, (or x Then these incrementalcapacitance signals (AC) can be fed to an operational differentiator foryielding electrical signals proportinal to the derivative (differencequotient) d(AC)/dx. Finally, the signals d(AC/dx) can be processed toyield a set of electrical signals proportional to the trap levelconcentration N, as a function of x (or x in accordance with equation(5) (or (9)).

While this invention has been described in detail in terms of deeplevels due to oxygen impurity atom in gallium phosphide, the profile ofmany other combinations of deep impurity levels in semiconductors can bemeasured using the above-described sequence of steps. Thus, the deeplevel impurity concentration of the various combinations may be measuredusing the abovedescribed techniques, such as deep level killer centers(whether identified chemically or not) in gallium arsenide, galliumphosphide; trap levels at interface of heterostructures of galliumarsenide gallium aluminum arsenide; deep level zinc-oxygen complexes ingallium phosphide; and deep level gold trap levels in silicon.

Although the invention has been described in terms of specificembodiments, many modifications are possible by the worker of ordinaryskill in the art without departing from the scope of the invention.

APPENDIX A In order to determine the profile of the net significantdonor and acceptor impurities (N and N in the neighborhood of thejunction (where the edges of the depletion regions are located), thefollowing method is useful. The technique described by J. A. CopelandIII in his U. S. Pat. No. 3,518,545 (issued on June 30, 1970) is firstused to determine (Nf N in this neighborhood as a function of 1,, (andthe corresponding x at various reverse voltage biases. In order todetermine separately the values of N, and N,, in the neighborhood of thejunction, the values of N A and N,, in regions removed from the junction10 can be found separately in P-type region and the N-type region inaccordance with the techniques described in said US. Pat. No. 3,518,545,by means of auxiliary Schottky barriers formed on an angle lapped(typically 2) side surface of the body 11. Typically, these Schottkybarriers are formed by metal contacts approximately 1 mil in diameter(whereas the width of the depletion region of the junction is typicallyless than a micron). Thereby, the asymptotic values of N and Np inregions removed from the junction are determined, and then theindividual values of N and N in the neighborhood of the junction 10 aredetermined as a function of position by asymptotically fitting theprofile of N A and N as functions of position, using the knowledge of(N, N,,) as a function of position in the neighborhood of the junction.

APPENDIX B In order to determine the value of r e /(e,.+e,,), a.

function of wavelength as defined in equation (6), for use in equation(9), the following procedure can be used. At fixed reverse voltage biasV,,, the body 11 is irradiated by source 21 with optical radiation ofwavelength A, sufiicient to fill the traps partially. Then the body 11is irradiated with wavelength A, from source 21, whereby all the trapsare emptied, while the capacitance of the body (and hence of junction10) is monitored by the capacitance meter 13 as a function of time atthe fixed bias V,,. The time constant for the capacitance to arrive atits new equilibrium value in the presence of the radiation A, fromsource 21 is equal to (I/e corresponding to A,. Thus, e,,-(A,) can befound as the reciprocal of this time constant. By using different valuesof A, (advantageously at fixed V, and A,) e,, can then be found as afunction of A, (i.e., for wavelengths which completely empty all traps).

Then, with all traps initially empty, the body 11 is irradiated withoptical radiation of wavelength A, from the optical source 21, suitablefor partially filling the traps, while the capacitance is againmonitored as a function of time. This time constant of the capacitanceto reach its new equilibrium value, in the presence of radiation A,, isequal to ll(e,,+e at A,. Thus, the value of (e,,+e, corresponding towavelength A, can be found as the reciprocal of this time constant. Byvary ing the value of A,, advantageously at fixed A, and V the values of(e-+e,,) can be determined as a function of A,.

it now remains to find ep itself as a function of A,. In order toaccomplish this, the curve of e as a function of A, (all traps empty) isextrapolated, by a trial approach, to the optical region of wavelengthsA, (traps partly empty). Thereby, a trial curve of e via A, can beplotted, and thence also a trial curve of r vs. A, can be plotted usingthe earlier found values of (e-+e, Then the capacitance of the body 11is'measured with all traps emptied (by suitable radiation A, from source21) as compared with the traps filled to the extent of the ratio r e/(e,,+e when irradiated with wavelength A, from source 21. Thedifference in these capacitance measurements (at fixed voltage bias) isequal to r e /(e,,+e to within a multiplicative constant. Thus, by usingdifferent values of A, (with fixed A, and voltage bias), anexperimentally found curve of Ar vs. A, can be plotted, where A is themultiplicative constant; so that the trial curve, if correct, should beidentical to this experimental curve for some value of themultiplicative (normalization) constant, A. If no such constant A can befound, that is, if no normalizatiion factor can be found to reconcilethe trial" curve of r vs. A,, then different trial extrapolations of thecurve of e vs. A, to optical regions of wavelengths A, are tried, untila multiplicative constant A can be found to reconcile the trial curve ofr vs. A, with the experimentally found curve of r vs. A,. Thereby, r canbe found both in optical regions of wavelengths A, (traps partly empty)and in regions of wavelengths A, (traps all empty).

What is claimed is:

l. A method for testing a semiconductor body which comprises:

a. applying a first voltage bias across the body;

b. irradiating the body under the first voltage bias, for

a first predetermined period of time;

c. decreasing the magnitude of the voltage applied across the body to asecond voltage bias and thereafter increasing the magnitude of thevoltage bias to the first voltage bias, to produce a first condition ofthe body; w

d. irradiating the body for a second predetermined period of time, toproduce a second condition of the body; and

e. measuring the change in electrical capacitance of the body in goingfrom the first to the second condition.

2. A method for testing a semiconductor body which comprises:

a. applying a first voltage bias across the body;

b. irradiating the body, under the first applied voltage, for a firstpredetermined period of time;

c. decreasing the magnitude of the voltage applied across the body to asecond voltage bias and thereafter increasing the magnitude of thevoltage applied across the body back to the first voltage bias, toproduce a first condition of the body;

d. irradiating the body for a second predetermined period of time, toproduce a second condition in the body; and

e. developing an electrical signal indicative of the difference inelectrical capacitance of the body going from the first to the secondcondition.

3. A method for testing the neighborhood of a P-N junction of asemiconductor, comprising the steps of:

a. applying a reverse bias voltage V across the P-N junction in order toproduce a depletion region in the neighborhood of the junction;

b. irradiating the junction with optical radiation of wavelength andintensity sufficient for producing a condition of the junctionthroughout the first depletion region produced by the bias voltage Vsaid depletion region characterized by a first predetermined ratio ofdeep levels which are empty of charge carriers to deep levels which arefilled with charge carriers;

c. varying the reverse bias voltage (in the absence of said opticalradiation) from V to a predetermined value V and then back to V,, saidvalue V corresponding to a second smaller width of the depletion region,in order that the junction be in' a second condition characterized by asecond predetermined ratio of deep levels which are empty of chargecarriers to deep levels which are filled on the N side of the P-Njunction in the semiconductor between the edges of the first and seconddepletion regions, and characterized by the first predetermined ratioelsewhere in the first depletion region;

d. irradiating the junction with optical radiation for returning thejunction into substantially the first condition; and

e. measuring the difference in electrical capacitance C, across thejunction going from the second condition to substantially the firstcondition.

4. A method for testing a semiconductor body which comprises the stepsof a. applying a reverse bias voltage V across the P-N junction in orderto produce a first depletion region having a first width in theneighborhood of the junction;

b. irradiating the junction with optical radiation of wavelength andintensity sufficient for producing a first condition of the junctionthroughout the first depletion region characterized by a firstpredetermined ratio of deep levels which are empty of charge carriers todeep levels which are filled with charge carriers;

. varying the reverse bias voltage (in the absence of said opticalradiation) from V to a predetermined value V .and then back to V saidvalue V corresponding to a second smaller width of the depletion region,in order that the junction be in a second condition characterized by asecond predetermined ratio of deep levels which are empty of chargecarriers to deep levels which are filled with charge carriers on the Nside of the P-N junction in the semiconductor between the edges of thefirst and second depletion regions and by the first predetermined ratioelsewhere in the first depletion region;

d. irradiating the junction with optical radiation for returning thejunction into substantially the first condition; and

e. developing a first electrical signal indicative of the differenceelectrical capacitance of the junction going from the second tosubstantially the first condition. v

5. The method recited in claim 4 which further comprises the steps ofrepeating the recited steps (a) through (e) with a different value of Vto develop a correspondingly different second electrical signal (S andthen developing a third electrical signal indicative of the differencein signals, S'S.

6. The method recited in claim 4 in which the body is essentiallygallium phosphide and the wavelength is between about 9,500 Angstromsand 13,500 Angstroms.

7. A method comprising the steps recited in claim 4 in which the body isessentially gallium phosphide which is maintained at a predeterminedtemperature to within 1 0.l C throughout the steps (d) and (e).

8. A method for testing a semiconductor body containing a PN junctionwhich comprises:

a. optically irradiating the body, under a first voltage bias appliedthereacross, for a first predetermined period of time to produce a firstcondition in the body, said irradiating being sufficient to effect anemptying of a predetermined proportion of deep impurity levels in agiven neighborhood of the P-N junction;

b. decreasing the magnitude of the voltage applied across the body to asecond voltage bias and thereafter increasing the magnitude of thevoltage applied across the body back to the first voltage bias, toproduce a second condition of the body; and

c. developing an electrical signal indicative of the change incapacitance of the body going from the first to the second condition.

9. A method for testing a semiconductor body containing a P-N junctionwhich comprises the steps of:

a. optically irradiating the body, under a first voltage bias appliedthereacross, for a first predetermined period of time to produce a firstcondition. in the body, said irradiating being sufficient to effect anemptying of a predetermined proportion of deep impurity levels in agiven neighborhood of the P-N junction; r

b. decreasing the magnitude of the voltage applied across the body to asecond voltage bias and thereafter increasing the magnitude of thevoltage applied across the body back to the first voltage bias, toproduce a second condition of the body; and

c. measuring the change in electrical capacitance of the body going fromthe first to the second condition.

10. A method for testing a semiconductor body comprising the stepsrecited in claim 9 in which the body is gallium phosphide which ismaintained at a predetermined temperature during the steps (b) and (c)to within 1- 0.l C.

i fi 1 8 i

1. A method for testing a semiconductor body which comprises: a.applying a first voltage bias across the body; b. irradiating the bodyunder the first voltage bias, for a first predetermined period of time;c. decreasing the magnitude of the voltage applied across the body to asecond voltage bias and thereafter increasing the magnitude of thevoltage bias to the first voltage bias, to produce a first condition ofthe body; d. irradiating the body for a second predetermined period oftime, to produce a second condition of the body; and e. measuring thechange in electrical capacitance of the body in going from the first tothe second condition.
 2. A method for testing a semiconductor body whichcomprises: a. applying a first voltage bias across the body; b.irradiating the body, under the first applied voltage, for a firstpredetermined period of time; c. decreasing the magnitude of the voltageapplied across the body to a second voltage bias and thereafterincreasing the magnitude of the voltage applied across the body back tothe first voltage bias, to produce a first condition of the body; d.irradiating the body for a second predetermined period of time, toproduce a second condition in the body; and e. developing an electricalsignal indicative of the difference in electrical capacitance of thebody going from the first to the second condition.
 3. A method fortesting the neighborhood of a P-N junction of a semiconductor,comprising the steps of: a. applying a reverse bias voltage VR acrossthe P-N junction in order to produce a depletion region in theneighborhood of the junction; b. irradiating the junction with opticalradiation of wavelength and intensity sufficient for producing acondition of the junction throughout the first depletion region producedby the bias voltage VR, said depletion region characterized by a firstpredetermined ratio of deep levels which are empty of charge carriers todeep levels which are filled with charge carriers; c. varying thereverse bias voltage (in the absence of said optical radiation) from VRto a predetermined value V and then back to Vr, said value Vcorresponding to a second smaller width of the depletion region, inorder that the junction be in a second condition characterized by asecond predetermined ratio of deep levels which are empty of chargecarriers to deep levels which are filled on the N side of the P-Njunction in the semiconductor between the edges of the first and seconddepletion regions, and characterized by the first predetermined ratioelsewhere in the first depletion region; d. irradiating the junctionwith optical radiation for returning the junction into substantially thefirst condition; and e. measuring the difference in electricalcapacitance C2 across the junction going from the second condition tosubstantially the first condition.
 4. A method for testing asemiconductor body which comprises the steps of a. applying a reversebias voltage VR across the P-N junction in order to produce a firstdepletion region having a first width in the neighborhood of thejunction; b. irradiating the junction with optical radiation ofwavelength and intensity sufficient for producing a first condition ofthe junction throughout the first depletion region characterized by afirst predetermined ratio of deep levels which are empty of chargecarriers to deep levels which are filled with charge carriers; c.varying the reverse bias voltage (in the absence of said opticalradiation) from VR to a predetermined value V and then back to VR, saidvalue V corresponding to a second smaller width of the depletion region,in order that the junction be in a second condition characterized by asecond predetermined ratio of deep levels which are empty of chargecarriers to deep levels which are filled with charge carriers on the Nside of the P-N junction in the semiconductor between the edges of thefirst and second depletion regions and by the first predetermined ratioelsewhere in the first depletion region; d. irradiating the junctionwith optical radiation for returning the junction into substantially thefirst condition; and e. developing a first electrical signal indicativeof the difference electrical capacitance of the junction going from thesecond to substantially the first condition.
 5. The method recited inclaim 4 which further comprises the steps of repeating the recited steps(a) through (e) with a different value of V to develop a correspondinglydifferent second electrical signal (S''), and then developing a thirdelectrical signal indicative of the difference in signals, S'' -S. 6.The method recited in claim 4 in which the body is essentially galliumphosphide and the wavelength is between about 9,500 Angstroms and 13,500Angstroms.
 7. A method comprising the steps recited in claim 4 in whichthe body is essentially gallium phosphide which is maintained at apredetermined temperature to within + or - 0.1* C throughout the steps(d) and (e).
 8. A method for testing a semiconductor body containing aP-N junction which comprises: a. optically irradiating the body, under afirst voltage bias applied thereacross, for a first predetermined periodof time to produce a first condition in the body, said irradiating beingsufficient to effect an emptying of a predetermined proportion of deepimpurity levels in a given neighborhood of the P-N junction; b.decreasing the magnitude of the voltage applied across the body to asecond voltage bias and thereafter increasing the magnitude of thevoltage applied across the body back to the first voltage bias, toproduce a second condition of the body; and c. developing an electricalsignal indicative of the change in capacitance of the body going fromthe first to the second condition.
 9. A method for testing asemiconductor body containing a P-N junction which comprises the stepsof: a. optically irradiating the body, under a first voltage biasapPlied thereacross, for a first predetermined period of time to producea first condition in the body, said irradiating being sufficient toeffect an emptying of a predetermined proportion of deep impurity levelsin a given neighborhood of the P-N junction; b. decreasing the magnitudeof the voltage applied across the body to a second voltage bias andthereafter increasing the magnitude of the voltage applied across thebody back to the first voltage bias, to produce a second condition ofthe body; and c. measuring the change in electrical capacitance of thebody going from the first to the second condition.
 10. A method fortesting a semiconductor body comprising the steps recited in claim 9 inwhich the body is gallium phosphide which is maintained at apredetermined temperature during the steps (b) and (c) to within + or -0.1* C.